The present invention relates to a means for calibrating the elevation and azimuth angles of the scan or radio axis of an antenna. It more particularly applies to the calibration of the elevation and azimuth angles of the scan axis of an antenna to be directed at a geostationary satellite occupying a predetermined position in space with respect to a fixed reference mark linked with the ground carrying the antenna.
FIG. 1 permits a better understanding of the definition of the elevation and azimuth angles of a point P in space occupying a predetermined position, said position being observed from a terrestrial point O.
The azimuth angle .theta. of a point P in space observed from a point O linked with the ground is the angle between a reference direction ON (e.g. the direction of the geographical north) and the straight intersection line OX between a vertical plane passing through P and through O and perpendicular to the horizontal ground. The elevation angle .beta. is the angle between the line passing through O and P and the aforementioned line OX.
Different processes or apparatuses exist for orienting the scan axis of an antenna with relatively great precision towards a point which, with respect to a fixed reference mark linked with the ground, occupies a predetermined position in space. These processes or apparatuses are particularly used for accurately directing the scan axis of an antenna on the predetermined position to be occupied by a geostationary satellite.
Different methods are known for performing angular elevation and azimuth measurements of a point in space and for orienting an antenna relatively accurately towards said point in space.
The angular elevation and azimuth measurements can be performed either by an optical method or by radio methods.
The optical method consists of using a telescope equipped with a camera which can e.g. photograph a satellite illuminated by the sun against a background of stars. The position of the satellite is detected by elevation and azimuth angles calculated from the positions of said satellite with respect to the stars, whose coordinates are perfectly known. This method is subject to constraints because viewing can only take place at the start and end of the night. Cloudiness must be very limited and it takes a long time to analyze the measurements.
Radio methods consist of using interferometry or performing a direct angular measurement. The method using interferometry consists, on the basis of the measurement of the phase displacements existing at all points of a particular antenna array, of restoring the direction from which the radio wave was transmitted by a source, whose elevation and azimuth angles it is wished to detect. With this direction detected, the antenna can then be directed towards the thus detected source. This method requires a large amount of equipment (several antennas with associated signal processing equipment). The method consisting of performing a direct angular measurement utilizes a directional antenna mounted on a support which moves along two orthogonal axes, e.g. the azimuth and elevation axes. It is easy to carry out this method, but its accuracy is dependent on the accuracy of the mechanical means which move the antenna.
The description of these various methods makes it clear that it is necessary to calibrate the measurements of the elevation and azimuth angles of the scan axis of an antenna, so that for each angle the angular error occurring during the measurements of said angles will be known, in order to correct the antenna direction when the antenna is directed at a point in space occupying predetermined positions in a fixed reference mark.
Different calibration methods exist. For example, it is known to use a collimation mast and this constitutes a widely used calibration method. In this method, use is made of a pylon which is provided with a radio beacon at its top, the transmission frequency of which corresponds to the reception frequency of the calibrated antenna. This pylon is also equipped with a sighting mark permitting its precise optical location from the antenna. It is easy to use, but it is only possible to calibrate the orientation of the antenna when the site is very low with all the resulting error sources and particular errors due to multiple reflections. It is not possible to use this process for calibrating an antenna with a limited displacement.
It is also possible to calibrate the measurements of elevation and azimuth angles of a radio axis of an antenna by using radio sources. The shortcoming of this method is that the radio source has an apparent diameter generally exceeding the sought measuring accuracy. Thus, the apparent diameter of a radio source is close to 2.10.sup.-3 radians, whereas the sought precision on directing an antenna at a geostationary satellite is close to 5.10.sup.-4 radians. This method cannot be used for an antenna with a limited displacement, due to the fact that the probability of finding a usable radio source in the antenna pointing direction is very limited.
It is also possible to carry out a calibration from a satellite. In this case, it is necessary to find a satellite having a beacon frequency which is identical or very close to the tracking frequency of the antenna to be calibrated. It is also necessary to know the position of the satellite with a much greater precision than the sought calibration precision. Finally, for an antenna with limited displacement, the problem mentioned in connection with a radio source occurs, i.e. that of having a satellite in the pointing direction and this is rarely the case.
The object of the present invention is to obviate the disadvantages of known devices for calibrating measurements of elevation and azimuth angles of the scan axis of an antenna and particularly aims at providing a calibration means for said angles which is much more accurate than existing means, which does not require the use of a large amount of equipment or extensive calculations and which makes it possible to very accurately calibrate the orientations of radio antennas with a limited displacement. The calibration accuracy obtained is less than 5.10.sup.-6 radians, which is much better than in existing equipment.